What sized gas port?

AR/M4 AR Technical Discussion
1

I’m adjusting the gas port size on a carbine length gas system M&P15 upper with the Black River Tactical gas port kit. The upper is mated to a Sionics Lower with an A5 RE and the standard A5 Buffer that comes from Sionics(A5H2, I believe). This is a bit of a franken gun, but I’m just looking to smooth up the upper and get rid of the overgassing with the kit from Black River Tactical. When the kit came today, it included inserts with .060, .063, and .067 ports, but no instructions.

Now, normally, I’d just google the size of a Colt 6920’s gas port and use whatever is closest, but I’m not familiar with how the A5 system will affect things. I understand it’s forgiving, but I’m not sure how much gas it will work best with from a carbine length gas system. Most of the ammo through this rifle will 223 pressure for target shooting and 3 gun, but plinking may include 5.56 bulk ammo. I plan on using PMC Bronze as my lowest pressure ammo in this upper. The purpose of this rifle is for my brother to get out shooting with me sooner than later, and we figured a Sionics A5 lower with a practically free M&P15 upper would be the quickest way to start at under $600. I’ve already shaved down the FSB, and will be adding an ALG rail in a couple days. The rifle will be scoped and used from 0-300 yards and might also be used to hunt coyotes in the MN/SD winter.

Should I just go with the .063, or will the A5 system do best with the smaller (or bigger) port?

2

.063" to start. If your S&W has a true 5.56 NATO chamber, it will work just fine.

3

Yes, the .063" should do nicely.

The A5 is a great system.

Gas Port sizes that work with H2 and rifle buffers seem to work well with the A5.

4

Much appreciated, gentlemen! Looks like this’ll come together nicely.

5

^ This guy might know a thing or two. :wink:

6

.063 is a very reliable go-to for carbine length gas systems. My A5 runs beautifully.

7

I believe that the S&W ports are larger. I know the last S&W midlength I saw was something like .081. A Colt .063 gas system will run with a Sprinco green spring and Vltor A5H3 buffer when shooting good ammo.

8

I’d say! We got the rifle together and it was much smoother than before. My calipers showed the gas port to be near .080. With the .063 port installed and the A5 system, it shot smoothly compared to the Colt 6920 I was shooting. And the 6920 shot smoothly compared to how that S&W upper used to shoot. I might have to throw a picture up on the custom build forum when I finish building the rifle I’m working on now. The 13" MLOK EMR really bring the rifle together well.

9

It’s amazing the difference proper gassing makes to the operation.

Thanks for the update.